1. Field of the Invention
The present invention relates to a substrate for a solar cell with a metal surface having formed thereon a zinc oxide film containing water in a prescribed amount or lower, a solar cell, and a production process of a solar cell.
2. Related Background Art
As a method for forming an oxide thin film by a vacuum process, well-known are an evaporation method, a sputtering method, a CVD method and the like. In the case of forming an oxide thin film by these methods, water adhering to the walls of a vacuum furnace is possibly taken into the film. However, any prior technique has not been described ever regarding the water content in a film in such a case.
Regarding the method for forming an oxide by intentionally introducing water molecules in the vacuum process, a relevant description is described in Japanese Patent Application Laid-Open No. 61-64874 document. According to the document, there is description that an oxide is deposited using argon gas mixed with water vapor as a sputtering gas, however the document has no particular description referring to the water content in the oxide film after deposition.
In place of the vacuum process, some methods (electrodeposition methods) for depositing oxide utilizing electrochemical reactions of aqueous solutions have been disclosed. For example, Japanese Patent Application Laid-Open No. 11-302896, titled, xe2x80x9cproduction method of an oxide thin filmxe2x80x9d discloses a method of producing an objective oxide by depositing a hydrate, a hydroxide, or a water-containing substance containing metal species a support and then changing the duty ratio of pulses of applied voltage to dehydrate while suppressing the dissolution. Here, existence of water in any form in the oxide deposited by the electrodeposition method is disclosed as a conventional technique. Further, there is also a description that dehydration is needed since the existence of water or the like deteriorates the reliability of the film. However, there is no description at all regarding the amount of the water contained in the objective oxide.
Further, Japanese Patent Application Laid-Open No. 10-140373 document discloses a method (an electro-deposition method) of forming a zinc oxide thin film on a support by applying an electric current in an aqueous solution produced by adding a carbohydrate to an aqueous solution containing nitrate ions and zinc ions. However, no description is given regarding the amount of water contained in the zinc oxide thin film.
Presuming that the usability of a substrate for solar cells depends on the amount of water contained in an oxide thin film, the present inventors have made an investigation of a zinc oxide film formed by a sputtering method, an investigation of a zinc oxide thin film involving water vapor introduction by a sputtering method, and an investigation of a zinc oxide film by deposition from an aqueous solution by an electrodeposition method.
As a result, in the case of a zinc oxide film produced by deposition from an aqueous solution by an electrodeposition method, it has been found that the electric resistance value of the film is not constant. Although the reason for that is not clear, when the resistance is changed by two or more orders of magnitude and becomes high, the initial characteristics of solar cells are deteriorated.
Also, in the case of making the surface area large in terms of industrial productivity, the uniformity, the adhesion and the like are sometimes insufficient.
The present invention has been accomplished in view of the above mentioned situation, and it is, therefore, an object of the present invention to provide a substrate for a solar cell having a zinc oxide film with optimal overall characteristic and a solar cell using the substrate.
According to a first aspect of the present invention, there is provided a substrate for a solar cell comprising a support having a metal surface and a zinc oxide film formed on the metal surface and having a water content of 7.5xc3x9710xe2x88x923 mol/cm3 or less.
In the present invention, it is preferable that the zinc oxide film has a water content of 4.0xc3x9710xe2x88x924 mol/cm3 or more and more preferable that the film has a water content of not less than 1.0xc3x9710xe2x88x923 mol/cm3 and not more than 5.0xc3x9710xe2x88x923 mol/cm3.
Further, it is preferable that the zinc oxide film is formed by electrodeposition utilizing an electrochemical reaction in an aqueous solution.
Moreover, it is preferable that the zinc oxide film is formed by sputtering in an atmosphere comprising water.
Also, it is preferable that the zinc oxide film is comprised of a plurality of layers.
In addition, it is preferable that the metal surface of the support comprises a metal selected from the group consisting of silver, aluminum, copper, silver alloy, aluminum alloy, and copper alloy.
According to a second aspect of the present invention, there is provided a process of producing a solar cell comprising the steps of:
forming a zinc oxide film on a support using an aqueous solution;
drying the zinc oxide film at a first temperature; and
forming a semiconductor layer on the zinc oxide film at a second temperature that is not higher than a temperature which is higher by 100xc2x0 C. than the first temperature. Incidentally, when a layer such as another semiconductor layer is further formed on the above-mentioned semiconductor layer, there is no restriction as to the formation temperature of the further formed layer (i.e., the layer which is not in contact with the zinc oxide film).
In the present invention, it is preferable that the second temperature is not higher than the first temperature.
Further, it is preferable that the first temperature is not lower than 200xc2x0 C. and not higher than 400xc2x0 C.
Presuming that problems in the above-described substrate for solar cells having a zinc oxide thin film significantly depends on the water content in the zinc oxide thin film, the inventors have carried out the following experiments.
A silver film with a thickness of 800 nm and successively a zinc oxide thin film with a thickness of 1 xcexcm were formed on a SUS 430 (2D surface) support at a set temperature of 200xc2x0 C. in argon atmosphere by a commercially available sputtering apparatus (manufactured by ULVAC, Inc.) (sample A).
Next, after a silver film was formed in the same conditions, the partial pressure of water was changed to be 5, 10, 15, and 30% and a zinc oxide thin film with a thickness of 1 xcexcm was formed (samples B to E). Further, the partial pressure of water was changed to be 0.1% and a zinc oxide thin film with a thickness of 1 xcexcm was formed (sample L).
Further, using a support on which a silver film was formed in the above-described conditions as a cathode and a zinc plate as an anode, a zinc oxide film with a thickness of 1 xcexcm was formed on the silver by an electrodeposition method in an aqueous zinc nitrate solution (sample F). Moreover, same samples F were dried under some different drying conditions to obtain samples (sample G to K).
In such a manner, several sheets of each samples were produced and their water contents were measured by Karl Fisher Moisture Titrator (MKC-510, manufactured by Kyoto Electronics).
Further, metals of Cr and successively Au, were evaporated on the zinc oxide thin film of each sample by a vacuum evaporation apparatus using a 0.25 cm2 mask and used as an upper electrode to measure the electric resistance value between the upper electrode and the SUS support. Since the measurement system including the measurement probe itself has the circuit resistance of about 0.1 xcexa9cm2, the electric resistance measurement was supposed to include measurement error to the extent of such a level.
Further, the total reflectivity and the irregular reflectivity of each sample were measured by a spectrometer (V-570, manufactured by JASCO Corporation Ltd.) in a range of 400 nm to 1,200 nm.
The measurement results (i.e., the relationship between the water content and the electrical resistance and reflectivity of zinc oxide thin films formed under various film forming conditions) are shown in Table 1. The water contents were determined by measuring the water amount per 8 cm2 of the substrate surface of a zinc oxide thin film and calculated the amount of water contained in the thin film (cm3) and expressed in number by mole. The values of the electric resistance were the values without correction containing the above-described error. The total reflectivity and the irregular reflectivity were the values measured at 800 nm wavelength. When the measurement results fluctuated, average values were calculated by drawing a curve contacting the hills and a curve contacting the valleys in the above-mentioned wavelength range and averaging the value of the curve contacting the hills and the value of the curve contacting the valleys at 800 nm wavelength and the thus obtained average values were used as the total reflectivity and the irregular reflectivity values.
Some facts are understood from Table 1. At first, in the case of the sample A produced by sputtering without water, the film after film formation was supposed to contain water although no water was introduced. That is, the water contained is probably attributed to the intake of the water adsorbed to the walls of a vacuum furnace and the water adsorbed to the thin film surface from the atmospheric air during the time until the water content measurement.
In the case of samples B to E for which water was added during the sputtering, it was found that the amounts of water taken into the zinc oxide thin films were increased stepwise, although slightly.
As compared with samples produced by sputtering, the sample F produced by the electrodeposition method (without drying) contained at least 10 times amount of water. The samples F to K produced by the electrodeposition method were found that the water content in the zinc oxide thin films could optionally be adjusted by changing the drying temperature and the drying time. In this case, the water content was decreased from the sample F to the sample J.
Regarding the reflectivity of each sample, the total reflectivity scarcely differed, whereas the irregular reflectivity increased with the increase of the water content and the irregular reflectivity reached 77 to 79% with the water content of approximately 1xc3x9710xe2x88x923 mol/cm3. The increase of the irregular reflectivity was supposedly attributed to that the transparency of a zinc oxide film was increased by oxygen (oxygen supposedly in form of water molecules and oxygen produced by decomposition of water during the film formation in the case of the sputtering method and oxygen supposedly almost all in form of water molecules in the case of the electrodeposition method) taken into the zinc oxide films during the film formation and that the morphology of the surface of the zinc oxide thin films was changed in the direction of proceed of surface unevenness due to water intake although the reason was unclear.
As a substrate for solar cells, the higher the irregular reflectivity is, the more Jsc is expected to be improved owing to the optical confinement effect. In the case of using silver for a metal layer on the substrate surface so as the case of the experiments, the optical confinement effect can be expected to be satisfactory when the irregular reflectivity is 70% or higher, in the case of samples containing a more amount of water, the Jsc is expected to be improved more owing to such an effect.
With the increase of the water content in a zinc oxide thin film, the value of the electric resistance is generally increased a little by a little. In the case of using the thin film for solar cells, those having electric resistance to a certain extent hardly cause shunt, so that the thin film is better to have a slight electric resistance rather than having no resistance. Especially, from a viewpoint that shunt hardly takes place under high temperature and high humidity environments, a remarkable effect can be expected.
However, in the case of the samples F and G, the values of the electric resistance were extremely high and if they are used as substrates for solar cells, the series resistance of the solar cells is possibly increased and the initial characteristics are probably considerably deteriorated. The upper limit values of the electric resistance are approximately 3.0 to 3.4 xcexa9cm2, which are of the sample H. For that, the water contents of the zinc oxide thin films are preferably 0.75xc3x9710xe2x88x922 mol/cm3.
After the sample A, the sample D, the sample J and the sample L shown in Table 1 were heated at 400xc2x0 C. for 30 minutes at 133 Pa (1 Torr) in hydrogen atmosphere in a vacuum furnace, they were taken out and the surface was observed with eyes. The sample D and the sample J were found unchanged before and after heating, whereas the sample A was found blackened in the zinc oxide thin film surface after heating. Further, as to the sample L, there were cases where the sample looked slightly blackened after heating. Although these were supposed to be used as solar cells without any problems, those containing more water were found more stable in the reducing atmosphere. That is, in the cases of the sample D, the sample J and the sample L containing a certain amount of water, practically 4.0xc3x9710xe2x88x924 mol/cm3 or higher, since the zinc oxide thin films were slightly oxygen-rich in relation to zinc, they are supposed to be chemically stable even in the reducing atmospheric conditions similar to these in the case of producing the solar cells (semiconductor films) on substrates and the optical properties are supposedly not deteriorated.
The zinc oxide thin film surfaces of the sample A, the sample B, the sample D, and the sample J shown in Table 1 were observed by an electron microscope. The unevenness of the surfaces was found increasing a little by a little with increase of the water content. Especially, regarding the sample J, as compared with other samples, the unevenness increased in a proceeding manner. That is, owing to the water content to a certain extent or more, those just like grain boundaries attributed to more water existing in the sample than in zinc oxide thin films produced by the vacuum process such as common sputtering, so that the film of the sample seemed to form a rugged shape seemingly composed of rocks laying. For that, a high irregular reflectivity was obtained and owing to the optical confinement effect, the Jsc improvement was more highly expected.
Only a zinc oxide thin film was formed on the sample A in the same manner as that of the sample J to give a sample M and a cross hatch test (JIS standardized) was carried out for the sample M and the sample J. The test results were 10 points for the sample M and 8 points for the sample J.
The experiments described above made the following clear: that the unevenness of the surface of the zinc oxide thin film proceeded and the irregular reflectivity increased with the increase of the content of water in the film. Further, with the increase of the water content in the zinc oxide thin film, the electric resistance increased a little b a little and thus the effect of making shunt difficult to take place could be expected. However, in the case the water content in a film was too high, the electric resistance was unstable or became high and thus the film was unsuitable for a substrate for solar cells.
Further, in terms of the adhesion, a double layer structure was more excellent than a single layer structure.
The inventors have completed the invention with the constitution as described above as a result of the enthusiastic investigations carried out to achieve the above described purposes, based on the above results of the experiments.
From the above-mentioned results, the water content of the zinc oxide film is preferably 7.5xc3x9710xe2x88x923 mol/cm3 or less, and more preferably 4.0xc3x9710xe2x88x924 mol/cm3 or more and 7.5xc3x9710xe2x88x923 mol/cm3 or less. Further, from the viewpoint of attaining a low resistivity and a high irregular reflectivity, it is preferable that the water content of the zinc oxide film is not less than 1.0xc3x9710xe2x88x923 mol/cm3 and not more than 5.0xc3x9710xe2x88x923 mol/cm3.
The following is the description of the support having a metal surface and an electrodeposition method to be employed for the embodiments of the invention.
Materials having a high optical reflectivity in the zinc oxide thin film forming surface are suitable for the support material to be employed for the invention. Further, in electrochemical deposition of zinc oxide film, many materials may be used as long as they have a metal surface which is capable of attaining electric conduction with the zinc oxide thin film forming surface and which is not corroded in an electrodeposition bath within a short time, and metallic materials such as SUS, Ag, Al, Cu, Fe and alloys thereof may be used. PET films coated with a metal are also usable. From these points of view, to be employed as a support for solar cells, those having silver, aluminum, copper, a silver alloy, an aluminum alloy, or a copper alloy in the zinc oxide thin film forming surface are especially excellent. Further, in consideration of the industrial productivity with surface area enlargement, in order to carry out device fabrication process in the succeeding steps, those produced by depositing silver, aluminum, copper, a silver alloy, an aluminum alloy, or a copper alloy on a long substrate made of a SUS are excellent.
A non-magnetic SUS and a magnetic SUS are both usable as the SUS. The typical example of the former is SUS 304, which is excellent in polishing property and is possible to have a mirror face of about 0.1 s. The typical example of the later is SUS 430, a ferrite type steel.
The surface of the SUS material may be smooth or roughened. The surface property is changed depending on the types of rolling rollers used in the rolling process of the SUS material. Those called as BA have an almost mirror face, while unevenness is remarkable in those called as 2D. In any surface, when observed with an SEM (scanning electron microscope), recesses with a size of a micrometer level are sometimes found significant. As a substrate for solar cells, rather than large undulation-like unevenness, a structure with a size of a micrometer level influences the characteristics of solar cells in both good and bad aspects.
The surface of silver, aluminum, copper, a silver alloy, an aluminum alloy, or a copper alloy may be smooth or roughened. As a substrate for a solar cells, when the surface has a proper unevenness structure of a micrometer level, the irregular reflectivity is expected to be improved. However, when the unevenness structure is too remarkable, the Voc of solar cells may sometimes be decreased or shunt may sometimes be induced, so that careful control is required. Further, when the metal surface is smooth, the unevenness, the shape, or the like is changed with an zinc oxide film, so that the irregular reflectivity is expected to be improved similarly to that described above. In this case also, if the roughened structure is too remarkable, the Voc of solar cells may sometimes be decreased or shunt may sometimes be induced, so that careful control is required.
A corrosion resistant container such as a beaker is filled with an aqueous electrodeposition solution and while the solution being stirring by a magnetic stirrer, a support having a metal surface is used as a cathode and a counter electrode is used as an anode and a DC power source is connected and a voltage is applied to the electrodes to form a zinc oxide thin film on the cathode.
The aqueous electrodeposition solution is an aqueous solution containing at least nitrate ions and zinc ions, and the concentration is preferably 0.002 mol/L (or mol/l) to 3.0 mol/L and more preferably 0.01 mol/L to 1.5 mol/L, and most preferably 0.05 mol/L to 0.7 mol/L. In such a manner, a zinc oxide thin film with a texture structure suitable for exhibiting the optical confinement effect can efficiently be formed.
Further, when saccharose or dextrin is added to the aqueous solution, the additive works so as to optimize the electrodeposition reaction to suppress abnormal growth of a zinc oxide thin film, so that the uniformity of the film forming surface can be kept excellent. In such a manner, a zinc oxide thin film with a texture structure and highly effective in the optical confinement effect can be formed at a high production yield. In the case of adding saccharose or dextrin as mentioned above, the concentration of saccharose is preferably 1 g/L to 500 g/L and more preferably 3 g/L to 100 g/L and the concentration of dextrin is preferably 0.01 g/L to 10 g/L and more preferably 0.025 g/L to 1 g/L.
The electric current to be applied between the support and the counter electrode is preferably 0.1 mA/cm2 to 100 mA/cm2 and more preferably 1 mA/cm2 to 30 mA/cm2 and most preferably 4 mA/cm2 to 20 mA/cm2.
Further, the pH of the solution is controlled to be 3 or more, the electric conductivity to be 10 mS/cm or more, and the solution temperature to be 60xc2x0 C. or more, so that a uniform zinc oxide thin film can efficiently be formed with scarce abnormal growth.